Spinel lithium titanate (Li4Ti5O12) is a popular anode material for a lithium battery in recent years. The material, which has a relatively high potential (1.55V) for lithium, can neither react with a common electrolyte to form a Solid Electrolyte Interphase (SEI) layer, nor have lithium precipitation when overcharged. Lithium titanate, with a change in crystal cell volume smaller than 1% during intercalation and deintercalation of lithium ions, is called as a “zero strain” material with an extremely long theoretical cycle life. However, lithium titanate has a poor electronic conductivity, thus easily resulting in a capacity decrease during charging and discharging at a high rate. Therefore, carbon coating methods have been widely applied in modified synthesis of a lithium titanate materials in order to improve the electronic conductivity of lithium titanate.
In-situ polymerization disperses inorganic particles in a polymer monomer uniformly, and then polymerizes the monomer in a certain condition to form a compound particle. A polymer may be coated uniformly on the surface of an inorganic nanometer particle via in-situ emulsion polymerization, and the polymerization only needs to undergo polymerization and molding once without subsequent treatment and processing. The surface of a particle, including TiO2, Al2O3, SiO2, etc. may be coated using an acrylate monomer through emulsion polymerization. However, modification of lithium titanate using emulsion polymerization is rarely reported.
CN101944590A discloses a preparation method of carbon-coated lithium titanate. A lithium source and a titanium source are dispersed, and then long-chain aliphatic carboxylic acid is added and heated to prepare a precursor, and sintering is performed under the protection of an inert gas subsequently, and obtaining carbon-coated lithium titanate. A surface carbon layer is obtained by means of surface coordination of long-chain aliphatic carboxylic acid and lithium titanate, thereby improving the conductivity of lithium titanate.
CN102593444A discloses a preparation method of carbon-coated lithium titanate and a product thereof. A certain amount of commercial lithium titanate and hexadecyl trimethyl ammonium bromide are weighed and dispersed in deionized water. Resorcinol, formaldehyde and sodium carbonate are added. A precursor obtained after a reflux treatment is calcined in an inert atmosphere, and then the carbon-coated lithium titanate is obtained. A carbon layer is obtained by coating phenolic resin on the surface of the lithium titanate, thereby improving the performance of the lithium titanate.
Carbon coating is applied to a modification method of a lithium titanate material in the foregoing two patents in order to improve the conductivity of lithium titanate. However, the precursor is prepared using a hydrothermal method in CN101944590A, which requires a high temperature and a high pressure with a complicated process. Besides, the carbon source is introduced by adding and stirring directly, thus uniformity and compactness can be hardly guaranteed. In CN102593444A, the lithium titanate is coated by phenolic resin and the polymer is mixed with the lithium titanate directly. Although the carbon source is combined with the lithium titanate more closely than common mechanical mixing, the polymer is mixed in the lithium titanate irregularly, which results in poor uniformity and fails to prevent agglomeration of the lithium titanate.